Dissolved air flotation (DAF) processes are known. Conventional DAF processes include injecting a coagulant into the water, mixing the coagulant with the water, injecting a flocculant into the water, mixing the flocculant in water, and then directing the coagulated and flocculated water to a zone where pressurized water, commonly referred to as white water, is injected and mixed with the water to be clarified. After the white water has been mixed with the coagulated and flocculated water, the water is directed to a separation zone which separates solids from the water through a dissolved air flotation process.
The basic principle of flotation for clarification is fundamentally the same regardless of the technology being used. By introducing gas or air to the water that is being treated, separation of solids from the water being treated is achieved. Gas bubbles are typically injected near the bottom of a basin which contains solids and flocculated particles to be separated from the liquid phase. Air bubbles attach to these solids via surface attachment and, in turn, cause the particles to rise to the surface. The floc particles that are formed and which have been floated to the surface form a discrete layer of sludge known as a “blanket” or “float”. This blanket of sludge can be removed by either hydraulic wasting or through mechanical scraping.
One of the drawbacks or shortcomings with a DAF process which utilizes chemicals is the cost to implement and operate such a process. An example of a dissolved air flotation system and a typical application may be helpful in appreciating the cost associated with chemical addition. For example, a wastewater plant may include a primary settler, a moving bed biological reactor (MBBR), and a dissolved air flotation system for removing suspended solids from the effluent from the MBBR unit. In this example, assume that the DAF unit is designed to treat 26,240 m3/day of wastewater. The DAF unit includes a coagulation stage, a flocculation stage, and a flotation stage. Chemicals such as a coagulant, for example ferric chloride, and a flocculant such as a polymer are injected into the water upstream of the flotation zone or stage. In this example, the velocity or mirror rate in the separation zone is about 7.6 m/h. The average efficiency for the removal of total suspended solids may be approximately 90%, thanks to the addition of the coagulant and flocculant. It has been generally observed, however, that when the chemicals are not added there is a substantial reduction in the total suspended solids removal efficiency.
Moreover, it is often challenging to properly implement the injection of coagulants and flocculants in dissolved air flotation processes. If not properly implemented, the total suspended solids removal efficiency in the DAF declines. For example, if there is too short of a contact time between the white water and the water being treated, then removal efficiency suffers. Likewise, if the velocity gradient (G) is not suitable, then the removal efficiency of suspended solids is reduced.
Conventional DAF systems are used throughout the world in a variety of ways. For example, they are used to treat drinking water for the removal of algae, floating materials, oil and disposal of substances responsible for color (humic and fulvic substance types), and colloidal substances present in raw water. In addition, conventional DAF systems are used in the desalination of sea water, pre-treatment of sea water for removing algae, oil, colloidal substances and particles responsible for clogging membranes. Finally, conventional DAF systems are widely used for removing suspended solids in the treatment of wastewater.
These conventional DAF systems and processes have drawbacks. First, they typically produce a low mirror velocity or rate, typically less than 10 m/h in the separation zone and hence, in the order to achieve desired capacity, the system must be designed to occupy a greater area. Further, conventional DAF systems require the preparation and conditioning of the substances to be removed by using specific stages of coagulation and flocculation in which chemical reagents are injected. As outlined above, the chemicals consumed are expensive and the energy consumed for driving mixers is costly.
Therefore, there is a need for a DAF system and process that is both effective and efficient without the need to add chemicals.